1. Field of the Invention
The present invention relates to apparatus for the noncontact measurement of the profile of a surface. More particularly, the invention relates to optical apparatus which is useful for the high accuracy measurement of surface roughness or of the height of a step change in thickness of an opaque film on a substrate.
2. The Prior Art
Prior art techniques available for measuring the profile of a surface include mechanical and optical profilers. A commonly used contacting apparatus used to measure surface profiles and step heights is a stylus instrument, e.g., the Talysurf or the Talystep. However, in the case of a soft or delicate surface, the stylus digs into the surface and measurement results do not truly represent the surface. Other limitations of the stylus technique include its high sensitivity to microphonics and vibrations, the delicate nature of the stylus and the mechanism, and the need for a highly skilled operator to align and use it.
There are numerous optical techniques available for measuring the profile of a surface. For a review and comparison of some of the more common techniques, see J. M. Bennett, "Comparison of Techniques for Measuring the Roughness of Optical Surfaces," Optical Engineering, Vol. 24, No. 3, pp. 380-387, 1985.
Prior art optical profilers have been based on a variety of techniques, e.g., scanning fringes of equal chromatic order (FECO) interferometry, see for example, J. M. Bennett, "Measurement of the RMS Roughness, Autocovariance Function and Other Statistical Properties of Optical Surfaces using a FECO Scanning Interferometer," Applied Optics, Vol. 15, pp. 2705-2721 (1976); scanning Fizeau interferometry, see for example, J. M. Eastman and P. W. Baumeister, "Measurement of the Microtopography of Optical Surfaces using a Scanning Fizeau Interferometer," J. Opt. Soc. Am., Vol. 64, p. 1369 (A) (1974); optical heterodyne interferometry, see for example, G. E. Sommargren, "Optical Heterodyne Profilometry," Applied Optics, Vol. 20, pp. 610-618, (1981); a Mirau interferometer, see for example, B. Bhushan, J. C. Wyant, and C. L. Koliopoulis, "Measurement of Surface Topography of Magnetic Tapes by Mirau Interferometry," Applied Optics, Vol. 24, pp. 1489-1497 (1985); a Nomarski-based instrument, see for example, S. N. Jabr, "Surface-roughness measurement by digital processing of Nomarski phase contrast images," Optics Letters, Vol. 10, pp. 526-528, (1985); a birefringent microscope, see for example, M. J. Downs, U.S. Pat. No. 4,534,649 issued Aug. 13, 1985; and shearing interference microscopy, see for example, M. Adachi and K. Yasaka, "Roughness measurement using a shearing interference microscope," Applied Optics, Vol. 25, pp. 764-768 (1986 ).
FECO interferometry requires that the surface under test be brought very close to the reference surface, e.g., typically within about several micrometers, thereby frequently causing the surface under test to be damaged by residual dust particles.
The optical heterodyne interferometer which is both common path and does not require a reference surface produces very accurate and precise measurements. While this technique provides state-of-the art optical measurements, it suffers from a number of limitations. In particular, the apparatus is complex and expensive. In addition, since the technique only scans in a circle of fixed radius, it does not profile an area of the surface under test.
The Mirau interferometer suffers from several serious limitations. First, since a beamsplitter and reference mirror must be placed between the objective lens and the surface under test, the resolving power, i.e., numerical aperture (NA), of this objecive lens is severely limited to about 0.60 NA, and the working distance of the objective lens is also reduced significantly. It is desirable to use the highest numerical aperture objective lenses in order to achieve the finest lateral resolution. Second, due to the presence of these optics between the objective lens and the surface under test, an extended light source is required. With a conventional light source, the coherence length is thusly limited to 3-6 micrometers. This short coherence length leads not only to a very tight angular alignment tolerance for the surface under test to obtain interference fringes, but also limits the heights of the steps which can be measured. In addition, the reference surface is in focus, thusly, adversely affecting the measurements.
The birefringent microscope technique is both common path and does not require a reference surface. However, it does have some severe limitations. First, it only scans a line so that it does not profile an area of the surface under test. Second, it is limited in its ability to use a sufficiently large diameter for the reference beam on the surface under test, thereby limiting the extent to which lower spatial frequencies can be measured.
In the present invention, high precision profile measurements can be made with the full range of objective lenses, i.e., from the lowest through the highest numerical aperture, with a large angular alignment tolerance, and with a large working distance over a line or an area of the surface under test. The improvements of the present invention, thusly, overcome the disadvantages of the prior art and allow the high accuracy, fine lateral resolution measurement of surface microroughness and step heights.